The Dali-Ruili Railway, a national key project, is known in the industry as one of the "hardest railways to build in the world." The Dali-Baoshan section opened to traffic on July 22, 2022, while the Baoshan-Ruili section, still under construction, includes the Gaoligongshan Tunnel, the world's first super project to cross the Hengduan Mountains.

The Gaoligongshan Tunnel, with a total length of 34.538 kilometers, is a key controlling project of the Dali-Ruili Railway. Since construction began in 2015, after nine years of strenuous effort by more than 1,000 builders, this super project—which has set nine records including China's longest railway tunnel, longest inclined shaft, and deepest vertical shaft—has completed 51.4% of its excavation.

Chinese railway builders have long been setting new world records in tunnel construction. Why has the Gaoligongshan Tunnel, less than 35 kilometers long, completed only half of its progress in nine years? What exactly makes it so difficult?

The Gaoligongshan Tunnel is located in the Himalayan seismic belt of the Hengduan Mountains, crossing a total of 19 fault zones with a maximum burial depth of 1,155 meters. It features the complex geology known as "three highs and four actives": high geothermal heat, high ground stress, and high seismic intensity; active neotectonic movement, an active geothermal water environment, active exogenic geological conditions, and active shallow-surface transformation of bank slopes.

"Industry experts unanimously agree that the Dali-Ruili Railway is currently the line with the most complex terrain and geological conditions in China's difficult mountainous areas, and the Gaoligongshan Tunnel is the most difficult among the difficulties on this line, with construction difficulty second to none in the world's tunnel engineering field today," said Ma Liang, chief engineer of the Dali-Ruili Railway Project Management Department of the China Railway Tunnel Group.

The tunnel passes through as many as 18 types of strata, a complexity rarely seen nationwide, encompassing almost all adverse geological conditions and major risks in tunnel construction. The construction difficulty is second to none in the history of world tunnel building. For the builders, working on it is like advancing through thorns, or boring a hole through a pile of sand.

Various superimposed and unimaginably complex geological problems—large deformation, massive water inflow, high temperatures, and collapses—have caused the builders immense hardship, and they have advanced almost inch by inch.

"When the inclined shaft was excavated to 600 meters, it entered a section of bedding-parallel strata, biased pressure, and high ground stress. We encountered the most dangerous and hardest-to-overcome large tunnel deformation since construction began. The high ground stress twisted the support steel frames into S and Z shapes, squeezed the already formed tunnel smaller, caused the initial support to crack and spall, and the wall rock intruded more than 1 meter into the tunnel, leaving a narrow, deformed working space," recalled Wang Xin, head of the engineering department of the inclined shaft work area at the Dali-Ruili Railway Project Management Department of the China Railway Tunnel Group. Inside the tunnel, initial support concrete was cracked everywhere, and tottering concrete blocks could fall at any time, testing the builders' willpower and constantly affecting excavation progress. Due to deformation and partial collapses caused by high ground stress alone, the project was halted 11 times in total.

To solve the large deformation problem, the participating units adjusted the curvature of the tunnel structure, strengthened the steel frame specifications, adjusted and tightened the steel frame spacing, applied radial grouting, and adopted measures such as forming the invert initial support and the arch-wall steel frames into a single ring at one time, effectively controlling surrounding rock deformation and resuming excavation. At the same time, relying on the Zhang Zheng Innovation Studio, they carried out extensive research on construction challenges, producing many papers and research results that provide important reference for similar large-deformation construction in soft rock in the future.

At the construction site, several workers sat on a pile of ice blocks at the tunnel face for a brief rest, bean-sized sweat drops mixed with melted ice water running down their backs.

Since July 2018, the temperature inside the tunnel has risen rapidly. The rock temperature at the tunnel face has exceeded 39°C at its highest, the inflow water temperature has reached 38°C, and the working area above the tunnel has reached as high as 42°C. There are hundreds of hot springs nearby, and deep-hole drilling has measured groundwater temperatures of up to 102°C. The high-temperature water inflow makes the construction site feel like a sauna at all times. The hot and humid environment causes builders to lose physical strength rapidly, greatly reducing work efficiency.

To address this, the project developed methods to block and intercept hot water sources, grouting to seal hot water outlets in the tunnel, and purchased high-power fans and two 450-kilowatt high-power mechanical refrigeration units to enhance ventilation, cooling, and air circulation. Considering the long-term need to cope with the high-temperature complex environment, the project invested 1.5 million yuan at once to build its own ice factory producing 60 tons of ice per day to meet the site's daily demand for nearly 60 tons of ice. It also equipped workers with an on-site medical room and emergency medicines, actively improving the working environment.

To improve construction efficiency, the project department designed two vertical shafts to add working faces and shorten the construction period. The main shaft of No. 1 vertical shaft of the Gaoligongshan Tunnel is 762.59 meters deep, and the auxiliary shaft is 764.74 meters deep, making it the deepest railway vertical shaft under construction in China. This "journey to the center of the earth" is especially difficult: during vertical shaft construction, mud bursts, water inflows, and collapses occurred many times, and at peak water inflow reached more than 300 cubic meters per hour, flooding the shaft.

"Data show that the inclined shaft alone has a daily water inflow of 33,000 cubic meters, enough to fill 17.5 standard swimming pools. At the same time, water pressure inside the Gaoligongshan Tunnel once reached 7 megapascals, setting the highest record for railway tunnel construction in China; drill bits 20 to 30 meters long were directly squeezed and flushed out of the rock by the high-pressure water inflow," said Wei Yongjie, chief engineer of the inclined shaft division at the Dali-Ruili Railway Project Management Department of the China Railway Tunnel Group. With every meter excavated, the builders face an uncontrollable and unknown world ahead; even with various advanced geological prediction methods combined, it is impossible to accurately predict the complex geology ahead.

To solve the high-temperature water inflow problem, they adopted the method of "drilling drainage holes to divert and drain water," drilling holes in advance to release water, which both avoids water inflow hazards and dissipates heat in advance. They invested in 450-kilowatt high-power pumping equipment for reverse-slope drainage to promptly remove high-temperature water inflow. They also carried out targeted research on key technologies for high-temperature, water-rich construction in ultra-long inclined shafts and on cooling treatment technologies, providing theoretical support for on-site cooling and drainage.

From the end of 2015 to the present, the builders have persisted in advancing the excavation progress of the "world's hardest railway tunnel to build," creating yet another miracle in the history of China's railway construction with their wisdom and sweat.

The Gaoligongshan Tunnel has set nine Chinese records: the world's first super project to cross the Hengduan Mountains; 24 routes were planned for the tunnel with 1,032 kilometers of route selection, making it the tunnel with the most route selections in China's railway history; with a design period of 10 years and a construction period of 10 years, it is the tunnel with the longest design and construction period in China; at 34.538 kilometers long, it was the longest railway tunnel in China at the time; the main shaft of No. 1 vertical shaft is 762.59 meters deep and the auxiliary shaft is 764.74 meters deep, making it the deepest vertical shaft in China's railways; No. 1 inclined shaft is 3,870 meters long, making it the longest inclined shaft in a Chinese railway tunnel; the advanced TBM method was used for the first time under complex geological conditions in southwestern Yunnan; the "Caiyun" TBM, with a diameter of 9.03 meters, is China's largest independently developed hard-rock boring TBM; and the "Caiyun No. 1" TBM is China's first remanufactured TBM.